There is always a temptation to craft ideal solutions to difficult problems and then to refuse to accept what is actually decided because it does not conform to the ideal. Soon, unless Osama bin Laden is handed over or it becomes clear he has left the country, people in western societies, and indeed people all over the world, will be faced with the reality of military action in Afghanistan. We can say now that it will not conform to the standards which have been set by anxious followers of American policy. This is not so much because the United States government is bent on some spectacular or excessive retaliation, since the evidence for that has, thankfully, been diminishing. It is that no military action could possibly meet the requirements that have been laid down.

A military response that will not anger Muslims, that will not endanger the political balance in Pakistan, that will not have unpredictable consequences in Afghanistan, and that will not be exploited by the jihadists and other extreme Islamist groups, is a chimera. Any military response must do these things to some degree. Then there are the further requirements that the response must be shaped to avoid an actual defeat for American and allied forces, must not inflict heavy casualties on Afghan civilians and must be organised in such a way that a failure to capture or kill Osama bin Laden will not be seen as a disaster.

Yet in the nature of these things, none of these objectives can be guaranteed. Those making the decisions in Washington have had, it would be fair to say, an impossible task, and what they come up with is bound to be imperfect. It has been right for Britain and America's other allies to argue for restraint, as perhaps Tony Blair is still doing. How much they were needed is not clear. The debate among the American policy makers, hidden but fierce, seems to have ended in victory for the relatively moderate side represented by Colin Powell and, in this instance, by Condoleezza Rice. It might have done so without European and other representations. But while being glad of this "moderate" outcome, it is obvious it could still lead to a military effort that could go very wrong.

Although it was absolutely right to make these representations, the moment is near when the time for consultation and playing some part in the decision making will be over. An operation or a series of operations will be going forward. They will be politically and militarily risky and they will play into the hands of the extremists, at least to some extent.

This may well be just what Osama bin Laden wants. But those who have serious doubts about the value of military action will have to ask themselves some hard questions. The first is whether it was remotely realistic to argue that there should be no military response at all at this stage. It is hyper-logical to argue that the people of the United States should completely still the natural impulse to move against those who caused such suffering. America is a country which tends to believe there is an answer to everything, a country which spends billions on its military forces, and a country which has been grievously hurt.

Even if, in an ideal world, the smartest thing to do might be to avoid military action at this juncture, we have to accept that a military response of some kind is necessary for America. In showing solidarity you cannot easily pick and choose, and if support of military action is at this moment what the Americans want, you cannot fend it off by saying you will introduce identity cards.

The second hard question is whether doubts about military action should be swallowed in the interest of promoting the long-lived American re-engagement with the world that now seems possible. There is in prospect an international programme which, with luck and good management, will attend not just to the immediate security of western countries, but to some of the world's fundamental ills. Many Americans, including some soldiers, are already aware of how limited a part conventional military force has to play. General Wesley Clark, the former Nato commander, recently spoke of the myriad forms of cooperation he believed will be necessary as having priority over strike operations.

The mass of arrangements with many countries which America has had to put together to make military action possible has been an exercise in multilateralism that is its own lesson for the Bush administration. That is equally true of the longer term cooperative action on security and other matters that is envisaged. Beyond that the realisation may be dawning that political problems left to fester are problems that get worse, and that no country can protect itself completely from the consequences. A government which had begun to think it would do everything important on its own discovered that the things that were most important to it could only be done with others. The United States, and other countries as well, may be moving to a re-ordering of priorities that has been long delayed but is all the more necessary for that reason.

It is true that some of the dangers of military action are really only aspects of dangers that already exist. It was already the case that the Taliban was driving its own battered population into even deeper despair and that the continued existence of this government had become a huge problem for the world, if only in terms of refugees. It was already true that Pakistan had bankrupted itself in vain pursuit of strategic leverage against India and had allowed its own extremists to grow in power to the point where the auguries for the next stage in its political life are not good.

We are left with the paradox that something very constructive may possibly come out of tragedy. But that possibility is unavoidably connected with a military operation that will throw up problems even if it is brief and successful, as we must hope it will be.